Various optical elements and devices, which are designed to guide and direct light in a desired direction by making use of a scattering phenomenon have been heretofore well known and used as back light sources for liquid crystal displays and so forth.
One type of these well-known optical elements and devices uses a two-dimensional light source comprising an elongated plate of a transparent material, wherein light is allowed to enter from one side of the plate of the transparent material and a reflective element is disposed on one surface of the plate while light-scattering property is provided on the other side surface to be used as a light exit surface; this light source is used, for example, as a backlight source for a liquid crystal display. This type is described for example, in Japanese Patent Laid-Open Nos. 235905/1987, 63083/1988, 13925/1990 and 245787/1990.
In the two-dimensional light source using such a light-scattering light-guiding devices, light is not scattered in three dimensions inside a transparent body, but is allowed to spread the direction of outgoing light, by utilizing irregular reflection or specular reflection near the surface of the transparent body or on the reflective element. Therefore, it is difficult, in principle, to sufficiently increase the proportion of scattered light which can be taken from the light-scattering light-guiding device.
Where a two-dimensional light source which introduces light from a side and has a uniform illumination intensity should be obtained, the reflecting power of the reflective element must have a certain gradient, as can be easily understood from the above-cited references. This makes the structure of the light-scattering light-guiding devices complex and bulky. Consequently, the manufacturing cost increases inevitably.
Thus, where this kind of light-scattering light-guiding device is used as a backlight source for a liquid crystal display or the like, some of various requirements such as brightness, the uniformity of the illumination intensity of the two-dimensional light source, thinness, and economy cannot help being sacrificed.
Another type of well-known optical element or device comprises an elongated plate of a transparent material and a particulate substance differing in the transparent material to form a light-guiding plate, the particulate substance is differing in refractive index from the transparent material.
For example, in those light-diffusing plates which are proposed in Japanese Patent Laid-Open Nos. 172801/1989, 207743/1991 and 294348/1989, spherical particles consisting of a silicone resin are dispersed in a transparent material such as PMMA or other resin. Using particulate diameters, particulate concentrations, and other data disclosed in the above-cited references, an effective scattering illumination parameter E (described later) is calculated at 210 to 3000.
These values of the parameter E deviate greatly from the range of from 0.45 to 100, which is a requirement of the present invention. Such values may be used for a light-diffusing plate in which light enters from a vertical direction, but cannot be used for a light-scattering light-guiding device such as one according to the present invention, in which the light of uniform brightness is required to be taken from a light exit surface having a spread.
More specifically, even if light is allowed to enter the above-described well-known light-scattering body from one side, such light-scattering body actually lacks in the function of a light-scattering light guide, i.e., the function to scatter light while guiding it over a considerable distance, with the result that only a limited area of the light exit surface very close to the light incident surface is locally made bright. In consequence, a uniform two-dimensional light source having a sufficiently practical size, i.e., several centimeters to several tens of centimeters, to be used as a backlight source for a liquid crystal display, for example, cannot be accomplished.
Attempts have been made to enhance the uniformity of the brightness of the light exit surface, as described in Japanese Patent Laid-Open Nos. 221924/1990, 221925/1990 and 221926/1990. According to these techniques, one or both of particulate diameter and particulate concentration is or are increased with increase in the distance from a light incident surface. This is equivalent to a scheme in which the value of the effective scattering illumination parameter E is increased with increase in the distance from the light incident surface. However, it is technically difficult to manufacture such a scattering body economically on mass production basis. For example, it is difficult to obtain a light-scattering light guide through a batch process by injection molding techniques.
Japanese Patent Laid-Open No. 145485/1992 discloses a light source comprising a light guide plate with a surface having a printed pattern to diffuse light, wherein the light guide plate consists of a transparent resin containing fine particles. It is described that the brightness at the exit surface of the light guide plate whose surface is not processed to diffuse light is set at 2 to 10% of the brightness of the exit surface of such a light guide plate whose surface is processed so as to diffuse light without containing particles in order to improve the brightness.
That is, the technical concept disclosed in the above-described reference is that more than 90% of the brightness derives from the light-diffusing processing on the surface; the remaining few percent increase of brightness is achieved by light diffusion owing to the inclusion of particles so that the scattering function of the light-scattering light guide itself is no more than an-auxiliary one.